1. Field of the Invention
The present invention relates to a bump bonding apparatus for forming by means of a wire bonding technique bumps for establishing an electrical connection to an electrode part of an IC chip. More specifically, the present invention relates to a bump bonding apparatus and method for forming such bumps on the wafer before being cut into IC chips.
2. Description of Related Art
Demand for smaller and lighter electronic devices has accelerated in recent years, particularly with respect to portable devices, and this has significantly increased demand for smaller IC chips for use in these electronic devices.
Conventionally, in a bump bonding apparatus, a single wafer is diced into a plurality of individual IC chips, and each IC chip is transported and positioned onto the bonding stage for bonding by ones, to form bumps at the electrode parts on the IC chip. Such bump bonding apparatuses, however, suffer from numerous technical problems, including: low production efficiency; difficulty in moving and handling the IC chips with a tray; and difficulty in positioning the IC chips with good precision.
Some of these problems can be addressed by forming bumps for individual IC chips before dicing the wafers into individual chips.
As described more fully below, however, forming bumps directly onto the undiced wafer presents a different set of technical difficulties as compared with handling the individual IC chips, and these problems must therefore be resolved in practice.
For example, while the wafers are supplied to the bump bonding apparatus in being stacked on multiple shelves in the wafer carrier, the wafers are loose in the carrier. Therefore, after the wafers are extracted from the carrier and regulated for its position on the positioning table, these wafers are moved to the bonding stage. The wafers are circular discs having a straight orientation flat cut into one edge of the wafer. On the positioning table, it is necessary to regulate a center of the wafer in position and regulate the circumferential location of the orientation flat. This requires a complex mechanism, such as a turntable and means for positioning the wafer from four directions around the wafer circumference. Positioning is also time consuming. Because the wafers are loose in the carrier, there is also the further danger of carrier vibration causing a wafer to fall out from its storage location.
When bumps are formed at various points around the entire surface of the stationary wafer with a large area, the relative movement distance of the bonding head and bonding stage becomes rather long. Therefore a longer arm is required for the bonding mechanism. This makes it mechanically difficult to assure sufficient positioning precision. These problems can be addressed by dividing the wafer circumferentially into a plurality of segments for bump formation, turning the wafer about its center when all bumps in one segment are formed, positioning the next segment for bump formation, and then forming the bumps in that segment.
In such case, if the bonding stage, on which the wafer is fixed, is turned, it heats up the stage. Thus thermal expansion and contraction of the turn drive mechanism makes consistent, high precision positioning difficult. This heat problem can be addressed in part by blowing a turn air flow to the bottom of the wafer to float and turn the wafer, and stopping the turn of the wafer based either on a timer or visual inspection by an operator. A problem with this method, however, is that minute variations in the turning air flow cause the wafer to move irregularly, easily producing variations in rotational positioning. Therefore, consistent, stable wafer positioning is difficult with the method.
Through extensive research focused on the various technical problems presented by directly forming bumps on an IC wafer before wafer dicing, the inventors of the present invention proposed, in Japanese Patent application No. 3-323064, a bump bonding apparatus and bump forming method that effectively resolves the above-noted problems.
However, while the above-noted bump bonding apparatus and bump forming method effectively solves the conventional problems described above, there remain a number of technical problems to be solved in order to yet further improve productivity, reduce cost, and improve quality.
With a conventional bump bonding apparatus, for example, the wafers are stored and supplied to the bump bonding apparatus in a stack on a plurality of shelves formed in the carrier. To make positioning the wafers on the bonding stage easier, a position regulating means is provided on the extractor for extracting the wafers from the carrier. A four-points chuck holds the wafer at four points after regulating the orientation flat to be oriented in a specific direction using the position regulating means, and moves the wafer to the bonding stage. Inserting the wafers in the carrier is a manual task performed by the operator. The operator must carefully insert each wafer in the carrier with the orientation flat oriented in a specific direction, so that the later positioning operation can proceed smoothly.
This means that the operator may need to repeatedly handle a wafer in order to position the orientation flat properly. In addition to making the operator work heavier, this also increases the potential for damage to high cost wafers. Further, in such case, the wafer is regulated for its position before transporting them to the bonding stage. This also increases the number of times for wafer handling, and thus increases the potential for wafer damage.
Also further, when the chuck holds the wafer at several points around the edge of the wafer for transport, it is also possible for one of the chucking points to be on the orientation flat of the wafer. If the chuck holds the wafer at only a few points, such as four points in the case of a 4-chucking points type chuck, the wafer may not be properly centered in the chuck. This means that despite efforts to regulate the wafer for its position, the orientation flat may become out of position, while the wafer is transported onto the bonding stage.
A sensor provided on the bonding stage is also used to detect the position of the orientation flat on the bonding stage in a conventional bump bonding apparatus. The bonding stage typically reaches temperatures of approximately 300 degrees centigrade. Therefore, expensive heat resistant sensors capable of withstanding such temperatures must be used for the orientation flat sensor, and this contributes to higher cost.
A conventional bump bonding apparatus also typically has an orientation flat sensor located on only one side of the bonding stage. This makes wafer positioning difficult when the wafer is divided circumferentially into plural segments for bump formation, and the wafer is sequentially turned about its center to form bumps in one section at a time. The air blowing means that is used for pushing the wafer to one side is also provided only in one direction, on which the orientation flat sensor is mounted. Furthermore, while only one orientation flat sensor is provided, a plurality of sensors is preferably provided along the base line of the orientation flat (the cut edge on the outside edge of the wafer) to improve the precision for detecting the orientation flat.
As also noted above, in a conventional apparatus, the method is proposed, wherein the wafer is divided circumferentially into a plurality of segments and is turned about its center to sequentially form bumps in each segment, and an air blowing means for floating and turning is therefore used. In this case, however, since the wafer has been heated on the bonding stage and is also cooled down by the air flow for floating, turning, or, pushing it to one side, the resulting rapid temperature change may adversely affect the wafer.
Yet further, when the wafer is floated on air for a turn above the bonding stage, consistently stable wafer turning is needed in order to increase orientation flat detection precision. Depending upon such factors as the wafer material and shape, however stabilizing wafer turning under specific conditions can be difficult with conventional technology. For example, when the surface roughness on the back side of the wafer exceeds a particular level, such as with quartz and lithium tantalate wafers, the friction coefficient of the back side of the wafer to the bonding stage surface is high and the wafer does not slide easily. This makes it necessary to use a relatively strong air flow, but when the air flow is increased, air flow turbulence is increased. Thus this makes it even more difficult to stabilize the wafer turning. A relatively strong air flow is also needed to start turning a heavy or large wafer. Increasing the air flow, however, also increases the effect of wafer inertia. This reduces the precision for stopping the wafer at the end of its turn, and thus leads to orientation flat detection error.
With consideration for the aforementioned problems of the related art, an object of the present invention is therefore to provide a bump bonding apparatus and bump formation method achieving a further increase in productivity, a reduction in cost, and an increase in quality when forming electrode bumps directly on a wafer before dicing it.